This invention relates to fuel vaporizers and, in particular, to fuel vaporizers to be used in fuel conditioners for supplying fuel gas to fuel cell powerplants.
Fuel cell powerplants are similar to conventional electric storage batteries in that, like electric storage batteries, fuel cell powerplants produce electrial current while consuming reacting chemicals at the powerplant electrodes. In storage batteries, the reacting chemicals are included in the battery electrodes themselves, and no transport of reactants to the electrodes is required. In contrast, the electrodes in fuel cell powerplants are inert, and the reactants must be transported to the electrodes from external sources.
Conventional types of fuel cell powerplants generally employ gases as reactants. Air is usually used as the cathode electrode reactant and hydrogen as the anode electrode reactant. The reactant hydrogen is commonly derived by steam reforming a liquid or gaseous hydrocarbon fuel. In this process the hydrocarbon fuel and water are first vaporized and a mixture of the hydrocarbon vapor and water vapor is then passed over a hot catalyst bed. The catalyst bed, in turn, converts the mixture to hydrogen and carbon dioxide for use as the anode electrode reactant. A fuel cell powerplant will thus generally include, in addition to a fuel cell, a fuel conditioner provided with a vaporizer and with a reforming catalyst bed for generating the anode or fuel reactant gas.
A requirement of fuel cell powerplants is that the rate of reactant flowing to the fuel cell electrodes be proportional to the current being drawn from the fuel cell. The reactant flow to the fuel cell is generally higher than the minimum rate predicted by Faraday's Law and, thus, all the reactant is not used in the electrochemical reaction. In the case of the anode or fuel reactant, the excess fuel not used for the electrochemical reaction is routed to a burner. The latter generates heat for the fuel vaporizer and the fuel reformer. For any given operating load both the fuel cell and the burner fuel requirements are fixed, thus fixing the rate of reformed fuel flow which must be delivered to the fuel cell from the fuel vaporizer and fuel reformer.
As can be appreciated, when the operating load of the fuel cell changes, the fuel requirements of the fuel cell and burner also change, thereby necessitating a change in fuel flow to the fuel cell in order to meet these changed requirements. A change in fuel flow to the fuel cell requires that there be a corresponding change in output fuel flow of the fuel vaporizer. While conventional vaporizers such as, for example, standard boilers, operate satisfactorly under constant output flow conditions, such vaporizers do not function adequately where changes in output flow are required. This is especially true where the changes in output flow must be rapid to accomodate rapid load changes, particularly rapid increases in output electric load from low to high values.
The inability of conventional boilers to rapidly vary output flow is caused primarily by the inertia of the boiler which delays production of vapor at the required changed rate. This delay results in the depletion of gaseous fuel at the fuel cell which, in turn, causes a loss of power and, thereby, decreased performance of the cell.
It is therefore an object of the present invention to provide a vaporizer for a fuel cell powerplant which is adapted to provide changes in output flow to accomodate rapid changes in loading of the fuel cell.